CHAPTER II. 



METHODS OF CULTIVATION OF BACTERIA. 



Introductory. In order to study the characters of any spe- 

 cies of bacterium it is necessary to have it growing apart from 

 every other species. In the great majority of cases where bac- 

 teria occur in nature, this condition is not fulfilled. In the gen- 

 eral processes of putrefaction many different species occur all 

 mingled with each other. Only in the blood and tissues in some 

 diseases do particular species occur singly and alone. We usu- 

 ally have, therefore, to remove a bacterium from its natural sur- 

 roundings and grow it on an artificial food medium. When we 

 have succeeded in separating it, and have got it to grow on a 

 medium which suits it, we are said to have obtained a pure cul- 

 ture. The recognition of different species of bacteria depends, 

 in fact, far more on the characters presented by pure cultures 

 and their behaviour in different food media, than on microscopic 

 examination. The latter in most cases only enables us to refer 

 a given bacterium to its class. Again, in inquiring as to the 

 possible possession of pathogenic properties by a bacterium, the 

 obtaining of pure cultures is absolutely essential. If two or 

 more different organisms be present together, we cannot say 

 that any one of them is the cause of the disease in question. 



To obtain pure cultures, then, is the first requisite of bacteri- 

 ological research. Now, as bacteria are practically omnipresent, 

 we must first of all have means of destroying all extraneous 

 organisms which may be present in the food media to be subse- 

 quently used for growing the bacteria we wish to study, in the 

 vessels in which the food media are contained, and on all instru- 

 ments which are to come in contact with our cultures. The 

 technique of this destructive process is called sterilisation. We 

 must, therefore, study the methods of sterilisation. The growth 

 of bacteria in other than their natural surroundings involves 

 further the preparation of sterile artificial food media, and when 

 we have such media prepared we have still to look at the tech- 



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